EXPERIMENT RUNS

The CSLM experiment was conducted on theMicrogravity Science Laboratory (MSL-1) mission, which flew as part of the STS-83 Shuttle flight. Since the mission was shortened from sixteen to four days, seven out of a planned nine experimental runs were completed. The runs that were performed had soak lengths (in seconds) of 36600, 14600, 5860, 2340, 940, 375, and 150. The runs that were not performed had soak lengths of 0 and 70 seconds, though the 70 sec run could have been switched to any of the other run times since it was set to run on sample processing unit (SPU) #9, which is the only SPU with a selectable run time.

The seven runs all ran to completion without major incident. However, there were unusual thermal oscillations near the end of the melting intervals in six out of the seven runs. These resulted in recorded temperature overshoots above the 185.0°C setpoint of up to 0.3°C for the control thermistor, and up to 1°C for other thermistors. The true cause of the temperature fluctuations remains unknown.

PRELIMINARY RESULTS FROM THE STS-83 EXPERIMENTS

In all the processed flight samples no significant segregation of the tin particles was found. On earth the tin particles float to the top of the samples due to their lower density. The tin particles were homogeneously distributed in the spaceflight samples (see Figure 5 ), with the distribution altered only at the surfaces due to temperature gradients (seen in all the samples except the center position specimens). Volume shrinkage due to solidification during the quench also rearranged particles, particularly in the samples with low volume content of tin particles, though this phenomenon was expected. The effect of the temperature gradients will be studied in more detail during the STS-94 mission.

Figure 5: Image showing tin particle dispersion in a 10 vol.% solid Sn sample across the entire sample cross-section (10mm dia.) after coarsening in microgravity at 185°C for 36600 sec (10 hr 10 min) aboard STS-83. Click here for a larger image (36 KB).

The arrangement of the particles in the samples is determined by the microgravity environment. When coarsening experiments are conducted on earth, all the tin particles move to the top of the samples until they are blocked by other particles in their way. This results in samples which are denuded of particles at the bottom, and at the top are stuffed (70-85% volume fraction) with particles (see Figure 6).

The process of analyzing the image data is continuing. Over 9300 pictures have been taken, and the particle size analysis has only just begun. The effect of the temperature gradients on the measured parameters needs to be determined before any final results are presented. The basic experimental technique of coarsening in a microgravity environment has been proven to work and good results are expected, with the samples from the center positions providing the best results.

Figure 6: Image showing the tin particle dispersion in a 10 vol.% solid Sn sample across the entire sample cross-section (10mm dia.) after coarsening on earth at 185°C for 36600 sec (10 hr 10 min). Click here for a larger image (20 KB).

PLANS FOR REFLIGHT ON STS-94

The MSL-1 mission will be reflown on STS-94, which is scheduled to launch on July 1, 1997 and land on July 17. Nine SPUs plus one ECU (electronic control unit) will be flown, and the experimental timeline for the reflight will be approximately the same. Most of the run times will change in order to provide data points between those obtained from the first flight samples, and a 24 hour run has been included. The seven SPUs that were used during the STS-83 mission have been refurbished, and some of the critical components have been replaced. The power control algorithm has been modified so as to minimize overshoot if any unusual temperature fluctuations occur. Some minor hardware and software modifications have been made that will permit more precise temperature measurements.

The new soak times are (in seconds) 86400(24 hours), 36600, 14600, 9575, 3835, 1535, 615, 150, and 70. The samples will nominally be the same as before, with a few exceptions. Only one grain boundary groove bicrystal will be used during one run (86400 sec). In the other runs the sample position that had been taken by the grain boundary groove bicrystal will have 15% solid volume fraction Sn samples, which were not used in the STS-83 experiments. To clarify the influence of the thermal gradients on the overall particle size distributions, two runs (the 36600 and 14600 sec runs) will be repeated from STS-83 with the sample from the center position swapped with a sample from an outside position. Since the sample in the center position was apparently unaffected by thermal gradients, it can be compared to a sample from the STS-94 run of the same composition and soak time but with the influence of the gradient. The same will be done for the sample that will be moved to the center position.